Re: [PR PATCH] [Merged]: [RFC] xbps-src: add -A flag for host arch, create masterdir per host arch

[the-maldridge <the-maldridge@users.noreply.github.com>]

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There's a merged pull request on the void-packages repository

[RFC] xbps-src: add -A flag for host arch, create masterdir per host arch
https://github.com/void-linux/void-packages/pull/46263

Description:
#### Testing the changes
- I tested the changes in this PR: **YES**

### Overview

- default masterdir naming now includes the host `XBPS_ARCH`, e.g. `masterdir-x86_64`
- setting the host `XBPS_ARCH` can be done with `-A <arch>`
    - This will create a new masterdir per host arch automatically
- using an arbitrary masterdir is still supported via `-m <masterdir>`, and overrides whatever `-A` would set
- the invocation of `[binary-]bootstrap` no longer (ab)uses `$XBPS_TARGET_PKG` for setting the masterdir's host architecture

#### Before
*default:*
```
$ ./xbps-src binary-bootstrap
$ ls
... masterdir ...
```
*nondefault masterdir:*
```
$ ./xbps-src -m masterdir-idk binary-bootstrap
$ ls
... masterdir-idk ...
```
*specific host arch:*
```
$ ./xbps-src -m masterdir-musl binary-bootstrap x86_64-musl
$ ls
... masterdir-musl ...
```
*cross with specific host arch:*
```
$ ./xbps-src -m masterdir-musl binary-bootstrap x86_64-musl
$ ./xbps-src -m masterdir-musl -a aarch64-musl pkg foo
```

#### After
*default:*
```
$ ./xbps-src binary-bootstrap
$ ls
... masterdir-x86_64 ...
```
*nondefault masterdir:*
```
$ ./xbps-src -m masterdir-idk binary-bootstrap
$ ls
... masterdir-idk ...
```
*specific host arch:*
```
$ ./xbps-src -A x86_64-musl binary-bootstrap
$ ls
... masterdir-x86_64-musl ...
```
*cross with specific host arch:*
```
$ ./xbps-src -A x86_64-musl binary-bootstrap
$ ./xbps-src -A x86_64-musl -a aarch64-musl pkg foo
```

### Thoughts
- ~~By removing the `[binary-]bootstrap [arch]` invocation style, there will be changes needed to buildbot configuration. If left in, should `-A <arch>` take priority over `[arch]` if both are specified?~~ buildbot config will be changed
- ~~This could be an opportunity to change the name of `masterdir` to something else, like `masterdir` (the containers CI uses are already called `void-buildroot-<libc>`)~~ if this changes, it will be separate; there's too much to change both coherently

### TODO
- [x] various documentation/other xbps-src changes as needed
- [x] buildbot changes if needed